| Glock internal locking rules |
| ------------------------------ |
| |
| This documents the basic principles of the glock state machine |
| internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h) |
| has two main (internal) locks: |
| |
| 1. A spinlock (gl_lockref.lock) which protects the internal state such |
| as gl_state, gl_target and the list of holders (gl_holders) |
| 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other |
| threads from making calls to the DLM, etc. at the same time. If a |
| thread takes this lock, it must then call run_queue (usually via the |
| workqueue) when it releases it in order to ensure any pending tasks |
| are completed. |
| |
| The gl_holders list contains all the queued lock requests (not |
| just the holders) associated with the glock. If there are any |
| held locks, then they will be contiguous entries at the head |
| of the list. Locks are granted in strictly the order that they |
| are queued, except for those marked LM_FLAG_PRIORITY which are |
| used only during recovery, and even then only for journal locks. |
| |
| There are three lock states that users of the glock layer can request, |
| namely shared (SH), deferred (DF) and exclusive (EX). Those translate |
| to the following DLM lock modes: |
| |
| Glock mode | DLM lock mode |
| ------------------------------ |
| UN | IV/NL Unlocked (no DLM lock associated with glock) or NL |
| SH | PR (Protected read) |
| DF | CW (Concurrent write) |
| EX | EX (Exclusive) |
| |
| Thus DF is basically a shared mode which is incompatible with the "normal" |
| shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O |
| operations. The glocks are basically a lock plus some routines which deal |
| with cache management. The following rules apply for the cache: |
| |
| Glock mode | Cache data | Cache Metadata | Dirty Data | Dirty Metadata |
| -------------------------------------------------------------------------- |
| UN | No | No | No | No |
| SH | Yes | Yes | No | No |
| DF | No | Yes | No | No |
| EX | Yes | Yes | Yes | Yes |
| |
| These rules are implemented using the various glock operations which |
| are defined for each type of glock. Not all types of glocks use |
| all the modes. Only inode glocks use the DF mode for example. |
| |
| Table of glock operations and per type constants: |
| |
| Field | Purpose |
| ---------------------------------------------------------------------------- |
| go_xmote_th | Called before remote state change (e.g. to sync dirty data) |
| go_xmote_bh | Called after remote state change (e.g. to refill cache) |
| go_inval | Called if remote state change requires invalidating the cache |
| go_demote_ok | Returns boolean value of whether its ok to demote a glock |
| | (e.g. checks timeout, and that there is no cached data) |
| go_lock | Called for the first local holder of a lock |
| go_unlock | Called on the final local unlock of a lock |
| go_dump | Called to print content of object for debugfs file, or on |
| | error to dump glock to the log. |
| go_type | The type of the glock, LM_TYPE_..... |
| go_callback | Called if the DLM sends a callback to drop this lock |
| go_flags | GLOF_ASPACE is set, if the glock has an address space |
| | associated with it |
| |
| The minimum hold time for each lock is the time after a remote lock |
| grant for which we ignore remote demote requests. This is in order to |
| prevent a situation where locks are being bounced around the cluster |
| from node to node with none of the nodes making any progress. This |
| tends to show up most with shared mmaped files which are being written |
| to by multiple nodes. By delaying the demotion in response to a |
| remote callback, that gives the userspace program time to make |
| some progress before the pages are unmapped. |
| |
| There is a plan to try and remove the go_lock and go_unlock callbacks |
| if possible, in order to try and speed up the fast path though the locking. |
| Also, eventually we hope to make the glock "EX" mode locally shared |
| such that any local locking will be done with the i_mutex as required |
| rather than via the glock. |
| |
| Locking rules for glock operations: |
| |
| Operation | GLF_LOCK bit lock held | gl_lockref.lock spinlock held |
| ------------------------------------------------------------------------- |
| go_xmote_th | Yes | No |
| go_xmote_bh | Yes | No |
| go_inval | Yes | No |
| go_demote_ok | Sometimes | Yes |
| go_lock | Yes | No |
| go_unlock | Yes | No |
| go_dump | Sometimes | Yes |
| go_callback | Sometimes (N/A) | Yes |
| |
| N.B. Operations must not drop either the bit lock or the spinlock |
| if its held on entry. go_dump and do_demote_ok must never block. |
| Note that go_dump will only be called if the glock's state |
| indicates that it is caching uptodate data. |
| |
| Glock locking order within GFS2: |
| |
| 1. i_rwsem (if required) |
| 2. Rename glock (for rename only) |
| 3. Inode glock(s) |
| (Parents before children, inodes at "same level" with same parent in |
| lock number order) |
| 4. Rgrp glock(s) (for (de)allocation operations) |
| 5. Transaction glock (via gfs2_trans_begin) for non-read operations |
| 6. i_rw_mutex (if required) |
| 7. Page lock (always last, very important!) |
| |
| There are two glocks per inode. One deals with access to the inode |
| itself (locking order as above), and the other, known as the iopen |
| glock is used in conjunction with the i_nlink field in the inode to |
| determine the lifetime of the inode in question. Locking of inodes |
| is on a per-inode basis. Locking of rgrps is on a per rgrp basis. |
| In general we prefer to lock local locks prior to cluster locks. |
| |
| Glock Statistics |
| ------------------ |
| |
| The stats are divided into two sets: those relating to the |
| super block and those relating to an individual glock. The |
| super block stats are done on a per cpu basis in order to |
| try and reduce the overhead of gathering them. They are also |
| further divided by glock type. All timings are in nanoseconds. |
| |
| In the case of both the super block and glock statistics, |
| the same information is gathered in each case. The super |
| block timing statistics are used to provide default values for |
| the glock timing statistics, so that newly created glocks |
| should have, as far as possible, a sensible starting point. |
| The per-glock counters are initialised to zero when the |
| glock is created. The per-glock statistics are lost when |
| the glock is ejected from memory. |
| |
| The statistics are divided into three pairs of mean and |
| variance, plus two counters. The mean/variance pairs are |
| smoothed exponential estimates and the algorithm used is |
| one which will be very familiar to those used to calculation |
| of round trip times in network code. See "TCP/IP Illustrated, |
| Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement", |
| p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards. |
| Unlike the TCP/IP Illustrated case, the mean and variance are |
| not scaled, but are in units of integer nanoseconds. |
| |
| The three pairs of mean/variance measure the following |
| things: |
| |
| 1. DLM lock time (non-blocking requests) |
| 2. DLM lock time (blocking requests) |
| 3. Inter-request time (again to the DLM) |
| |
| A non-blocking request is one which will complete right |
| away, whatever the state of the DLM lock in question. That |
| currently means any requests when (a) the current state of |
| the lock is exclusive, i.e. a lock demotion (b) the requested |
| state is either null or unlocked (again, a demotion) or (c) the |
| "try lock" flag is set. A blocking request covers all the other |
| lock requests. |
| |
| There are two counters. The first is there primarily to show |
| how many lock requests have been made, and thus how much data |
| has gone into the mean/variance calculations. The other counter |
| is counting queuing of holders at the top layer of the glock |
| code. Hopefully that number will be a lot larger than the number |
| of dlm lock requests issued. |
| |
| So why gather these statistics? There are several reasons |
| we'd like to get a better idea of these timings: |
| |
| 1. To be able to better set the glock "min hold time" |
| 2. To spot performance issues more easily |
| 3. To improve the algorithm for selecting resource groups for |
| allocation (to base it on lock wait time, rather than blindly |
| using a "try lock") |
| |
| Due to the smoothing action of the updates, a step change in |
| some input quantity being sampled will only fully be taken |
| into account after 8 samples (or 4 for the variance) and this |
| needs to be carefully considered when interpreting the |
| results. |
| |
| Knowing both the time it takes a lock request to complete and |
| the average time between lock requests for a glock means we |
| can compute the total percentage of the time for which the |
| node is able to use a glock vs. time that the rest of the |
| cluster has its share. That will be very useful when setting |
| the lock min hold time. |
| |
| Great care has been taken to ensure that we |
| measure exactly the quantities that we want, as accurately |
| as possible. There are always inaccuracies in any |
| measuring system, but I hope this is as accurate as we |
| can reasonably make it. |
| |
| Per sb stats can be found here: |
| /sys/kernel/debug/gfs2/<fsname>/sbstats |
| Per glock stats can be found here: |
| /sys/kernel/debug/gfs2/<fsname>/glstats |
| |
| Assuming that debugfs is mounted on /sys/kernel/debug and also |
| that <fsname> is replaced with the name of the gfs2 filesystem |
| in question. |
| |
| The abbreviations used in the output as are follows: |
| |
| srtt - Smoothed round trip time for non-blocking dlm requests |
| srttvar - Variance estimate for srtt |
| srttb - Smoothed round trip time for (potentially) blocking dlm requests |
| srttvarb - Variance estimate for srttb |
| sirt - Smoothed inter-request time (for dlm requests) |
| sirtvar - Variance estimate for sirt |
| dlm - Number of dlm requests made (dcnt in glstats file) |
| queue - Number of glock requests queued (qcnt in glstats file) |
| |
| The sbstats file contains a set of these stats for each glock type (so 8 lines |
| for each type) and for each cpu (one column per cpu). The glstats file contains |
| a set of these stats for each glock in a similar format to the glocks file, but |
| using the format mean/variance for each of the timing stats. |
| |
| The gfs2_glock_lock_time tracepoint prints out the current values of the stats |
| for the glock in question, along with some addition information on each dlm |
| reply that is received: |
| |
| status - The status of the dlm request |
| flags - The dlm request flags |
| tdiff - The time taken by this specific request |
| (remaining fields as per above list) |
| |
| |